It is common in variable displacement hydraulic units to have a hydraulic servo control mechanism for varying unit displacement. A low pressure source of control fluid, such as a charge pump in a transmission, provides control fluid to the servo mechanism through a displacement control valve. For many variable displacement hydraulic units, this low pressure source, generally in the neighborhood of 200 to 300 psi, provides sufficient fluid pressure to operate the hydraulic servos with proper response speeds under normal operating conditions.
However, some variable displacement hydraulic units, such as modern design bent axis variable displacement motors which change displacement by sliding a valve plate segment in the endcap, have high internal friction especially in a stall condition when there is no shaft rotation. Such units, when stalled require relatively high servo forces to move the valve plate segment. This necessitates either large servo pistons, which are more expensive and increase unit volume, or a source of high pressure. Therefore, some hydraulic units are provided with relatively small servos which are designed to utilize unit main loop working pressure as the control fluid source. When high pressure sources are utilized to provide control fluid, unacceptable response speed changes occur relative to variable system working pressures. This is especially true in the case of dual path systems where both transmission motors must react together in a coordinated, controlled manner. Furthermore, a high pressure source for control fluid may require additional or modified pumps and will increase power loss during normal operation.
Motor pressure compensating systems have been used wherein main loop working pressures are applied to the motor servos only upon an overpressure condition. These systems do not have any low pressure control fluid source and the displacement is not varied other than in overpressure conditions. Pump pressure limiter systems generally have a displacement control utilizing a low pressure source, such as charge pressure, and sense high working pressure to provide an additional control signal upon an overpressure condition, but such additional control signal is limited by valves to also be of a low pressure nature. Units such as taught in U.S. Pat. No. 4,480,963, Ring issued Nov. 6, 1984, use the highest of main loop pressures and charge pressure. Therefore, the pump servos are always subjected to the highest pressure available and thus cannot have a response rate substantially independent of working pressure. The latter is also true of units such as taught in U.S. Pat. No. 4,194,364, Pahl et al. issued Mar. 25, 1980.